It is hard to explain what a brush is, but mainly it is just a 3d structure. A wall for instance can be 1 brush. Just a large stretched cube.
A brush has only 1 thing it cannot have. A "convex" corner (i think it is called convex, don't shoot me if it is the wrong term. What I mean by that is that if you would want an L shaped brush, you would have to use 2 brushes to get the effect. Why? Because the inner side of the L cannot be done by using only 1 brush. I know my explanation kinda sucks, but I have no other way how to explain it.
A simple room. Just create six brushes, modify their height,width and lenght, and their position, so that you create a large cube in 3D. Put a player_info_start in it, and a light, compile it and voila, you finished your first map.